Treatment of magnesium and magnesium alloy articles to increase their resistance to corrosion



Patented Mar. 29, 1949 FICE Rene Gide, New York, N. Y.

No Drawing. Application August 3, 1945, Serial No. 608,841

8 Claims.

The invention relates to the treatment of metal articles to increase their resistance to corrosion. It has special application to the treatment of magnesium and its alloys to form a corrosion resistant coating for such metals, but is useful also in the treatment of other metals and alloys.

It is an object of my invention to provide an improved method of treating metals and metal articles to increase their resistance to corrosion.

I have discovered that improved resistance to corrosion of metals, for example, magnesium, aluminum, and iron, and alloys in which these metals form the principal ingredient, can be obtained by treating in at least one boiling solution comprising about 1 to 6 per cent of potassium permanganate and about 0.1 to 1 per cent of a salt of a metal which is lower in the electromotive series than the metal to be protected. The protective value of the coating thereby produced can be further enhanced or the stability of the coating increased by a subsequent treatment which consists of dipping the articles in fuming oil.

I have discovered, further, that an especially effective coating can be produced by treating successively in two boiling solutions each of which comprises about 1 to 6 per cent of potassium permanganate, the first solution containing also a sulphate such as a sulphate of aluminum, manganese or zinc, and the second containing in addition to the permanganate, a salt of a metal which is lower in the electromotive series than the metal which is being treated.

The percentages of the components of the boiling solutions are of importance in achieving optimum results.

My improved treatment preferably is preceded by cleaning the surface of the metal article to be protected. This may be accomplished mechanically, chemically, or electrochemically according to well known methods which form no part of the present invention. However, by way of example, removal of the oxides on the surface of magnesium and its alloys can be carried out by immersion in a 20 per cent chromic acid solution.

In some cases it is beneficial to follow the cleaning step just described with an electrolytic degreasing step in which the work is made cathode in a suitable solution. Such treatments are well known in the art, and these known treatments may be practiced as desired, or may be omitted.

In the principal treating step or steps of my method, I immerse the metal or metal articles to betreated in at least one boiling solution which, asI have stated, comprises about 1 to 6 per cent of potassium permanganate and 0.1 to 1 per cent of a salt or salts of a metal which is lower in the electromotive series than the metal being treated. I prefer to employ in succession, two boiling solutions each containing potassium permanganate within the limits stated. In this preferred treatment, the first solution will contain, in addition to the permanganate, a sulphate such as aluminum, manganese, or zinc sulphate. The second solution will contain in addition to the permanganate, 0.1 to 1 per cent of a sulphate, or acetate of zinc, cobalt, or nickel. Where two solutions are used successively, it may be preferable to use in the first solution a salt of a metal which is just slightly lower in the electromotive series than the metal to be protected, and to use in the second solution a salt of a metal which is lower in the electromotive series than the metal of the salt used in the first solution. For example, assuming that the metal to be protected is magnesium, the first solution may contain aluminum sulphate and the second, zinc acetate. Note that in this instance the salt used in the first solution is both a sulphate and a salt of a metal which is lower in the electromotive series than the metal to be protected. However, the sulphate used in the first solution may in some cases be of a metal which is higher in the electromotive series than the metal to be protected, being followed as it is by treatment in a second solution containing a salt of a metal lower in the series. On the other hand, it is essential in all cases that the work be treated in at least one solution containing both potassium permanganate and a salt of a metal lower in the series. Articles made of dissimilar metals are amenable to the treatment, as the radical ions cover one of the metals and the metal ions the other.

While I have described, as my preferred method, one which comprises successive treatments in the two boiling solutions, I have found that a very satisfactory coating can be produced by treatment in a single solution. The preferred single solution will contain potassium permanganate and both the sulphate and the salt of a metal which is lower in the electromotive series, or potassium permanganate and the sulphate of a metal which is lower in the series. If desired, more than one sulphate may be used. For example, two sulphates such as manganese and zinc sulphate may be used in combination. Also, the solution may contain more than one salt of metals which are lower in the electromotive series.

Other constituents can be added to the treating solution as may be desired for special purposes, for examplefto impart a particular color or texture to the protective coating or otherwise to vary its appearance or character in respects which do not substantially alter its corrosion resistant properties. For example, small amounts of selenic acid may be employed in the treatment of magnesium by either the single solution method or the two solution method I have described. 1 have found that selenic acid, when added in small amounts up to about 0.05 per cent, imparts a dark reddish or mahogany color and gives a very smooth surface. In the two solution method, the selenic acid would be added to the second of the two solutions described.

The salt of the metal which is lower in the electromotive series than the metal to be protected preferably is a sulphate, although acetates may also be used. Thus, in the single sluti0n method, assuming that the metal to be protected is magnesium, aluminum sulphate, manganese sulphate or zinc sulphate might be used. These same salts are to be preferred for use in the second solution of the two-solution method, but manganese acetate or zinc acetate (for example) might be used instead.

Treatment in the boiling solutions may be continued for a period of to 30 minutes or longer. In the two-solution method, treatment in the first solution for minutes usually is satisfactory, and the time in the second solution may be on the order of 10 to minutes or longer. In most cases, a period of minutes is sufficient for the entire treatment, whether performed in one solution or in two solutions successively. However, as the solutions used do not cause any appreciable dimensional change, the time of treatment can be extended if desired. The work may be washed between successive immersions, but that is not necessary. At the conclusion of the treatment, the articles are washed in hot water and allowed to dry. If desired, they are then dipped in fuming oil, for example, in refined corn oil, for a period of a few seconds to one minute. During this treatment, some evolution of gas will be observed, and at such time as this evolution ceases the treatment can be terminated.

Finally, the articles are washed in hot soapy Water and allowed to dry, or de-oiling can be performed by immersing in a boiling aqueous solution containing 2 per cent potassium permanganate and 0.1 per cent of a sulphate such as aluminum or zinc sulphates. In the case of magnesium alloys, the coating varies in color from black to gray or maroon, according to the particular alloy treated. The coating grows blacker as the percentage of aluminum in the alloy in-- creases. Thus a magnesium alloy which does not contain aluminum takes a maroon color while a magnesium alloy containing 9 per cent of aluminum takes a dark gray to black color.

The coatings produced by the treatments I have described are very adherent and can be brushed heavily with a hard bristle brush without any apparent harm to the coatings.

Example 1 By way of specific example of the practice of my invention, whereby its nature and advantages may be more fully understood, I shall now describe the treatment of a magnesium alloy casting containing about 6 per cent aluminum, 0.2 per cent manganese and 3 per cent zinc, balance substantially magnesium plus usual impurities. The casting was cleaned and degreased, and then immersed for a period of 30 minutes in a boiling ill '3 ing in a boiling aqueous solution containing 2 per cent potassium permanganate and 0.1 per cent zinc sulphate. Upon removal from the second treating solution, the casting when dried had a rusty color. When dipped in the fuming oil, evolution of gas was observed and the surface of the casting took on a black appearance. The treated casting was tested by immersion in a 20 per cent sodium chloride solution for 1260 hours, at the end of which time the coating was found to be hard, dense and smooth. The casting was only slightly corroded, notwithstanding the severity of this test.

Example 2 A die casting made of a magnesium alloy containing about 9 per cent aluminum, 0.2 per cent manganese and 0.3 per cent zinc, balance substantially magnesium plus usual impurities, was subjected to the same treatment as described in Example 1. A smooth, hard, adherent coating of brownish black color was produced. The treated casting was tested by immersion in a 20 per cent sodium chloride solution for 250 hours, and showed remarkable resistance to corrosion.

Emam'mle 3 A magnesium alloy casting containing about 6 per cent aluminum, 0.2 per cent maganese and 3 per cent zinc, balance substantially magnesium plus usual impurities, was subjected to the same treatment as described in Example 1. A smooth, hard coating of black color was produced. The treated casting was tested by immersion in a 20 per cent sodium chloride solution for 4'70 hours, at the end of which time the coating was still smooth and hard, and only slight evidence of corrosion was to be observed.

Example 4 A magnesium alloy casting of the same composition as given in Example 3 but having a small piece of aluminum alloy sheet attached thereto by means of a steel stud bolt, was subjected to the same treatment as described in Example 1. The coating on the aluminum and magnesium parts was black and the coating on the iron was of a brownish or brownish-black color. The treated magnesium-aluminum-iron assembly was tested by immersion in a 20 per cent sodium chloride solution for 470 hours. It was found that all three metals withstood corrosion remarkably well, and that normal galvanic action had been efiectively arrested.

Example 5 A specimen of rolled sheet of an aluminum alloy containing about 1.2 per cent copper, '7 per cent silicon and 0.3 per cent magnesium, balance substantially aluminum plus usual impurities, was subjected to the same treatment as described in Example 1. A smooth, hard, adherent coating of dark yellow color was produced.

Example 6 A specimen of rolled sheet of a magnesium alloy containing about 6.15 per cent aluminum, 0.2 per J cent manganese and 0.7 per cent zinc, balance substantially magnesium plus usual impurities, was treated as follows: Immersed for a period of minutes in a boiling aqueous solution containing 2 per cent potassium permanganate and 0.25 per cent aluminum sulphate. After removal from this solution, it Was then immersed for a period of 15 minutes in a boiling aqueous solution containing 2 per cent potassium permanganate and 0.1 per cent zinc sulphate. Upon removal from the second solution, the specimen was dipped in fuming oil and then cleaned or de-oiled by Washing in a boiling aqueous solution containing 2 per cent potassium permanganate and 0.1 per cent cobalt sulphate. The color of the coating produced was bluish gray.

Example 7 A specimen of rolled sheet of a magnesium alloy containing about 2.7 per cent aluminum, 0.3 per cent manganese and 1 per cent zinc, balance substantially magnesium plus usual impurities, was subjected to the following treatment: Immersed for a period of minutes in a boiling aqueous solution containing 2 per cent potassium permanganate and 0.25 per cent aluminum sulphate. After removal from this solution, it was then im mersed for a period of minutes in a boiling aqueous solution containing 2 per cent potassium permanganate and 0.1 per cent zinc sulphate. On removal from the second solution, the specimen was dipped in fuming oil and then cleaned by washing in hot soapy Water. A coating of brownish-black color was obtained, this coating having the characteristic hardness, smoothness and. adherence to which reference has been made previously.

Example 8 A specimen of rolled sheet of a magnesium alloy containing about 6.15 per cent aluminum, 0.2 per cent manganese and 0.7 per cent zinc, balance substantially magnesium plus usual impurities, was treated by immersion for a period of minutes in a boiling aqueous solution containing 2 per cent potassium permanganate, 0.5 per cent aluminum sulphate and 0.1 per cent zinc sulphate. After removal from this single solution, the specimen was clipped in fuming oil producing a smooth, dark brown, adherent coating. lhe treated. specimen was tested by immersion in a 20 per cent sodium chloride solution for 100 hours, at the end of which time it was found to be smooth and uncorroded.

Example 9 A specimen of fiat bar steel containing about 0.30 per cent carbon, 0.50 per cent manganese and 0.90 per cent chromium, balance substantially iron plus usual impurities, was treated by immersion for 45 minutes in a boiling aqueous solution containing 2 per cent potassium permanganate and 0.5 per cent cobalt sulphate. The fuming oil treatment was not employed. The specimen took on a dark mottled bronze color.

Example 10 A specimen of rolled sheet of magnesium alloy containing about 6 per cent aluminum, 0.3 per cent manganese and 1 per cent zinc, balance substantially magnesium plus usual impurities, was treated as follows: Immersed for a period of 20 minutes in a boiling aqueous solution containing 1 per cent potassium permanganate and 0.1 per cent aluminum sulphate. After removal from this solution, it was then immersed for 10 minutes in a solution containing 1: percent potassium permanganate and 0.1 per cent zinc sulphate. Upon removal from the second solution, the specimen was dipped in fuming refined-com oil and then cleaned or de-oiled by washing in a boiling aqueous solution containing 2 per cent potassium permanganate and 0.1 per cent zinc sulphate. A smooth. hard coating, gray to black, was produced.

Eaxample 11 A specimen of rolled sheet of the magnesium alloy described in the preceding example was treated as follows: Immersed for a period of 15 minutes in a boiling aqueous solution containing 6 per cent potassium permanganate and 1 per cent aluminum sulphate. After removal from this solution, it was then immersed for 5 minutes in a solution containing 6 per cent potassium permanganate and 1 per cent zinc sulphate. Upon removal from the second solution, the specimen was dipped in fuming refined corn oil and then cleaned or de-oiled by washing in a boiling aqueous solution containing 2 per cent potassium permanganate 0.1 per cent zinc sulphate. A smooth, hard coating, of brownish black color, was produced.

Example 12 A specimen of rolled sheet of the magnesium alloy described in Example 10 was treated as follows: Immersed for a period of 20 minutes in a boiling aqueous solution containing 6 per cent potassium permanganate and 0.1 per cent aluminum sulphate. After removal from this solution, it was then immersed for 10 minutes in a solution containing 6 per cent potassium permanganate and 0.1 per cent zinc sulphate. Upon removal from the second solution, the sheet was dipped in fuming refined corn oil and then cleaned or deoiled by washing in a boiling aqueous solution containing 2 per cent potassium permanganate and 0.1 per cent zinc sulphate. A smooth, hard coating, of brownish black color, was produced.

Example 13 A specimen of rolled sheet of the magnesium alloy described in Example 10 was treated as follows: Immersed for a period of 15 minutes in a boiling aqueous solution containing 1 per cent potassium permanganate and 1 per cent aluminum sulphate. After removal from this solution, it was then immersed for 5 minutes in a solution containing 1 per cent potassium permanganate and 1 per cent zinc sulphate. Upon removal from the second solution, the specimen was dipped in fuming refined corn oil and then cleaned or de-oileol by washing in a boiling aqueous solution containing 2 per cent potassium permanganate and 0.1 per cent sine sulphate. A smooth, hard coating, of dark brown color, was produced.

It will be appreciated from the preceding description and examples that my improved treatment is extremely simple in character and can be practiced without the use of special equipment. Moreover, it requires only a short time to produce an erfective coating of smooth and pleasing appearance, the color of which can be varied. Another advantage or" my treatment is that it is carried out without the use of acids or other substances which are dangerous to handle. The treatment is amendable to sheet, castings or articles made of dissimilar metals.

The terms and expressions which I have employed are used in a descriptive and not a limiting sense, and I have no intention of excluding 0.1 to 1 per cent of a salt of a metal which is lower in the electromotive series than the metal to be protected, and about 1 to 6 per cent of potassium permanganate, removing the articles from said solution and dipping them in fuming hot oil, and

de-oiling the articles by washing in a solvent for the oil.

2. The method of treating magnesium and magnesium base alloy articles which comprises the steps of cleaning and degreasing the articles, and immersing the articles in a boiling aqueous solution effective to increase their resistance to corrosion, said solution comprising about 0.1 to 1 per cent of a salt of a metal which is below magnesium in the electromotive series, and about 1 to 6 per cent of potassium permanganate.

3. The method of treating magnesium and mag nesium base alloy articles which comprises the steps of immersing the articles in a boiling aqueous solution comprising primarily about 0.1 to 1 per cent of zinc sulphate, and about 1 to 6 per cent of potassium permanganate, removing the articles from said solution and dipping them in fuming hot oil, and de-oiling the articles by washing in a solvent for the oil.

4. The method of treating magnesium and magnesium base alloy articles which comprises immersing the articles in a boiling aqueous solution efiective to increase their resistance to corrosion, said solution comprising about 0.1 to 1 per cent of a sulphate and about 1 to 6 per cent of potassium permanganate, and thereafter immersing the articles in a second boiling aqueous solution comprising about 0.1 to 1 per cent of a salt of a metal which is lower in the electromotive series than the metal to be protected and about 1 to 6 per cent of potassium permanganate.

5. The method of treating magnesium and magnesium base alloy articles which comprises immersing the articles in a boiling aqueous solution comprising about 0.1 to 1 per cent of a sulphate and about 1 to 6 per cent of potassium permanganate, thereafter immersing the articles in a second boiling aqueous solution comprising about 0.1 to 1 per cent of a salt of a metal which is lower in the electromotive series than the metal to be protected and about 1 to 6 per cent of potassium permanganate and which is effective to increase resistance to corrosion, removing the articles from the second solution and dipping them in fuming 8 hot oil, and de-oiling the articles by washing in a solvent for the oil.

6. The method of treating magnesium and magnesium base alloy articles to increase their resistance to corrosion which comprises cleaning and degreasing the articles, and immersing the articles in a boiling aqueous solution comprising primarily a sulphate and salt of a metal which is lower in the electromotive series than the metal to be protected, and potassium permanganate, the content of potassium permanganate in the solution being about 1 to 6 per cent, and the total content of the other salts aforesaid being about 0.1 to 1 per cent.

'7. The method of treating magnesium and magnesium base alloy articles to increase their resistance to corrosion which comprises immersing the articles in a boiling aqueous solution comprising primarily about 0.1 to 1 per cent of a sulphate and about 1 to 6 per cent of potassium permanganate, and thereafter immersing the articles in a second boiling aqueous solution comprising about 0.1 to 1 per cent of a salt of a metal which is below magnesium in the electromotive series and about 1 to 6 per cent of potassium permanganate.

8. The method of treating magnesium and magnesium base alloy articles to increase their resistance to corrosion which comprises immersing the articles in a boiling aqueous solution comprising primarily about 0.1 to 1 per cent of an aluminum sulphate and about 1 to 6 per cent of potassium permanganate, and thereafter immersing the articles in a second boiling aqueous solution comprising about 0.1 to 1 per cent of a salt of a metal which is below magnesium in the electromotive series and about 1 to 6 per cent of potassium permanganate. RENE GIDE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,247,086 Crowe Nov. 20, 1917 1,710,743 Pacz Apr. 30, 1929 1,965,269 Tosterucl July 3, 1934 2,077,450 Weisberg et al Apr. 20, 1937 2,127,206 Curtin Aug. 16, 1938 2,138,023 Buzzard Nov. 29, 1938 2,219,554 Batcheller Oct. 29, 1940 2,438,740 Clark et al. May 30, 1948 FOREIGN PATENTS Number Country Date 510,353 Great Britain July 24, 1939 

